Cancer is a leading cause of death worldwide. In the United Stated alone, cancer accounts for about 500,000 deaths per year, and the American Cancer Society estimates that in 2010 approximately 1.5 million new cases of cancer were diagnosed. Cancer treatment modalities include chemotherapy, biologics, external beam radiation and surgical excision. Traditional cancer chemotherapy is nonspecific and targets all rapidly dividing cells, resulting in undesirable side effects. In addition, tumors can become resistant to chemotherapy. Biologics afford greater selectivity, but these agents can stimulate an immune response. For some subjects, the symptoms may be relatively minor, but for others, the symptoms are severe and may result in an inability to continue treatment. Moreover, because biologics are large molecules, they are more challenging and costly to manufacture and/or formulate than small molecule agents, and they generally must be administered intravenously in a clinical setting. External beam radiation and surgery can only target known tumor sites and are ineffective against undetectable metastatic lesions. There is a continuing need for chemotherapeutic agents that selectively target tumor cells.